Structural modelling of the complex of leucyl-tRNA synthetase and mis-aminoacylated tRNA

To assure fidelity of translation, class Ia aminoacyl-tRNA synthetases (aaRSs) edit mis-aminoacylated tRNAs. Mis-attached amino acids and structural water molecules are not included simultaneously in the current crystal structures of the aaRS•tRNA complexes, where the 3′-ends (adenine 76; A76) are bound to the editing sites. A structural model of the completely solvated leucyl-tRNA synthetase complexed with valyl-tRNA^Leu was constructed by exploiting molecular dynamics simulations modified for the present modelling. The results showed that the ribose conformation of A76 is distinct from those observed in the above-mentioned crystal structures, which could be derived from structural constraints in a sandwiched manner induced by the mis-attached valine and tRNA^Leu.     

Identification of the nucleophilic factors and the productive complex for the editing reaction by leucyl-tRNA synthetase

To ensure fidelity of translation, several aminoacyl-tRNA synthetases (aaRSs) possess editing capability to hydrolyse mis-aminoacylated tRNAs. In this report, based on our previously-modelled structure of leucyl-tRNA synthetase (LeuRS) complexed with valyl-tRNA^Leu, further structural modelling has been performed along with molecular dynamics simulations. This enabled the identification of the nucleophile, which is different from that suggested by the crystal structure of the LeuRS • Nva2AA complex. Our results revealed that the 3′ hydroxyl group of A76 acts as a “gate” to regulate the accessibility of the nucleophile; thus, the opening of the gate leads to the productive complex for the reaction.     

Two E3 ubiquitin ligases, SCF-Skp2 and DDB1-Cul4, target human Cdt1 for proteolysis

Replication licensing is carefully regulated to restrict replication to once in a cell cycle. In higher eukaryotes, regulation of the licensing factor Cdt1 by proteolysis and Geminin is essential to prevent re-replication. We show here that the N-terminal 100 amino acids of human Cdt1 are recognized for proteolysis by two distinct E3 ubiquitin ligases during S-G2 phases. Six highly conserved amino acids within the 10 first amino acids of Cdt1 are essential for DDB1-Cul4-mediated proteolysis. This region is also involved in proteolysis following DNA damage. The second E3 is SCF-Skp2, which recognizes the Cy-motif-mediated Cyclin E/A-cyclin-dependent kinase-phosphorylated region. Consistently, in HeLa cells cosilenced of Skp2 and Cul4, Cdt1 remained stable in S-G2 phases. The Cul4-containing E3 is active during ongoing replication, while SCF-Skp2 operates both in S and G2 phases. PCNA binds to Cdt1 through the six conserved N-terminal amino acids. PCNA is essential for Cul4- but not Skp2-directed degradation during DNA replication and following ultraviolet-irradiation. Our data unravel multiple distinct pathways regulating Cdt1 to block re-replication.     

Regulation of mitochondrial proteostasis by the proton gradient

Mitochondria adapt to different energetic demands reshaping their proteome. Mitochondrial proteases are emerging as key regulators of these adaptive processes. Here, we use a multiproteomic approach to demonstrate the regulation of the m‐AAA protease AFG3L2 by the mitochondrial proton gradient, coupling mitochondrial protein turnover to the energetic status of mitochondria. We identify TMBIM5 (previously also known as GHITM or MICS1) as a Ca²⁺/H⁺ exchanger in the mitochondrial inner membrane, which binds to and inhibits the m‐AAA protease. TMBIM5 ensures cell survival and respiration, allowing Ca²⁺ efflux from mitochondria and limiting mitochondrial hyperpolarization. Persistent hyperpolarization, however, triggers degradation of TMBIM5 and activation of the m‐AAA protease. The m‐AAA protease broadly remodels the mitochondrial proteome and mediates the proteolytic breakdown of respiratory complex I to confine ROS production and oxidative damage in hyperpolarized mitochondria. TMBIM5 thus integrates mitochondrial Ca²⁺ signaling and the energetic status of mitochondria with protein turnover rates to reshape the mitochondrial proteome and adjust the cellular metabolism.     

Gift-giving in romantic couples serves as a commitment signal: Relational mobility is associated with more frequent gift-giving

The present study explored why married couples periodically exchange gifts. Based on the commitment signal hypothesis, we tested whether relational mobility, which was operationalized as divorce rate in Study 1 and relational opportunities in Study 2, is positively correlated with the frequency of gift exchanges among married couples. In Study 1, we found that married couples in the U.S., which is associated with a relatively high divorce rate, were more likely to give and receive gifts to and from their partners than those in Japan, which is associated with a relatively low divorce rate. This societal difference, in contrast, was not observed among unmarried couples, who were still developing their relationship (and thus, partner changes could frequently happen regardless of the country-level divorce rate). Study 2, a secondary analysis of extant survey data, revealed that Japanese married couples who have more relational opportunities more frequently engage in gift exchanges than those who do not. Together, these results support our hypothesis that periodical gift exchanges work as commitment signals among married couples.     

Differential Expression of an Auxin-Regulated Gene, parC, and a Novel Related Gene, C-7, from Tobacco Mesophyll Protoplasts in Response to External Stimuli and in Plant Tissues

As part of an extended search for auxin-regulated genes otherthan parA and parB in the cDNA library from cultured tobaccomesophyll protoplasts, we have isolated the cDNA for a genedesignated parC. This gene is expressed during the transitionfrom the G₀ to S phase of the cell cycle. The nucleotide sequenceof parC cDNA was similar to that of parA. Using the parC cDNAas a probe we have isolated cDNA for a gene designated C-7 byhybridization at reduced stringency. Even though C-7 is relatedto parC, as is parA, its mode of expression was, to our surprise,completely different from that of parC. The C-7 gene is predominantlyexpressed in mature leaves and in freshly prepared protoplasts,but its level of expression did not change in response to auxintreatment. Although parC and C-7 are related to one anotherand exhibit homology to the parA gene, the two former genesdemonstrated conspicuous differences in their responses to externalstimuli, which included auxin, as well as their tissue-specificexpression. These results provide us an interesting system forthe analysis of the differential expression of closely relatedgenes. The significance of our observations is discussed withreference to the other members of the family of parA genes. (Received May 8, 1992; Accepted June 17, 1992)     

A new biosynthetic route of porphyrin precursors in common between animals and plants

An additional enzyme, 4-oxo-5-hydroxyvalerate (OHV) dehydrogenase was identified and characterized. This enzyme catalyzes the conversion of OHV to 4,5-dioxovalerate, a direct precursor of 5-aminolevulinate. The enzyme was partially purified from rat liver supernatant as two isoenzyme (ca. 40,000 and 70,000 dalton). 5-Aminolevulinate was formed from OHV via 4,5-dioxovalerate by this dehydrogenase and alanine-4,5-dioxovalerate aminotransferase (EC 2.6.1.43). This dehydrogenase required NADP of NAD as a hydrogen acceptor. The enzyme was heat sensitive and catalyzed the reaction reversibly. The dehydrogenase was present in the high speed supernatants of liver and kidney of rat, rabbit and human, and that of spinach leaf.     

PARL partitions the lipid transfer protein STARD7 between the cytosol and mitochondria

Intramembrane‐cleaving peptidases of the rhomboid family regulate diverse cellular processes that are critical for development and cell survival. The function of the rhomboid protease PARL in the mitochondrial inner membrane has been linked to mitophagy and apoptosis, but other regulatory functions are likely to exist. Here, we identify the START domain‐containing protein STARD7 as an intramitochondrial lipid transfer protein for phosphatidylcholine. We demonstrate that PARL‐mediated cleavage during mitochondrial import partitions STARD7 to the cytosol and the mitochondrial intermembrane space. Negatively charged amino acids in STARD7 serve as a sorting signal allowing mitochondrial release of mature STARD7 upon cleavage by PARL. On the other hand, membrane insertion of STARD7 mediated by the TIM23 complex promotes mitochondrial localization of mature STARD7. Mitochondrial STARD7 is necessary and sufficient for the accumulation of phosphatidylcholine in the inner membrane and for the maintenance of respiration and cristae morphogenesis. Thus, PARL preserves mitochondrial membrane homeostasis via STARD7 processing and is emerging as a critical regulator of protein localization between mitochondria and the cytosol.     

Isolation and Characterization of a Cytokinin Up-Regulated Gene from Tobacco Mesophyll Protoplasts

We have isolated a cytokinin up-regulated cDNA clone, H13, froman early stage of cultured tobacco mesophyll protoplasts bya differential display method. The expression of this gene wasspecifically induced by natural and synthetic cytokinins includingN-(2-chloro-4-pyridyl)-N'-phenylurea (4PU30), a diphenylurea-typecytokinin, although the simultaneous presence of auxin was alsorequired. It seems that the preceding treatment of the tobaccomesophyll protoplasts by auxin is necessary for the gene torespond to cytokinin. The addition of a cytokinin antagonist,compound 182, which suppressed the induction of cell divisionin tobacco mesophyll protoplasts, completely abolished the expressionof this gene. Though the predicted gene product of H13 did notsuggest us any sequences of defined functions, two domains ofthe predicted sequence had significant homology to several reportedsequences in the data base. The gene product of H13 is proposedto have a role in regenerating cell wall in cultured protoplasts,since a cDNA clone E6, from cotton fiber cells, which has themost closely related structure to H13, has been isolated fromcells which showed active cellulose synthesis. This suppositionis supported by the evidence that in the absence of cytokinin,cell wall regeneration was significantly suppressed, resultingin failure of the induction of cell division. Thus, the geneproduct of H13 is supposed to have a role in regenerating cellwalls and facilitating the progression of the cell cycle, resultingin the sustained cell division of tobacco mesophyll protoplasts. ¹These authors are equally contributed to this work.